Some integrated circuit chips are fabricated with exposed, small metal contacts or pads on the chips so that the chips may be placed on a substrate, such as a wafer or a circuit board, for direct connection in a joinder process known as "solder bumping". This is in contrast to an older form of chip mounting involving wire leads extending from packaged chips, wherein the leads fit into holes on the board or into sockets mounted on the board.
Solder bump joinder of chips to substrates tends to preserve rising and falling edges of waveforms. This benefit arises because in the transmission of data between chips discontinuities associated with impedance mismatches are minimized. The solder bump technique also allows chips to be directly connected to heat dissipating supports. In U.S. Pat. No. 4,558,812 to A. S. Bailey et al., chip carriers are held in cavities by vacuum. A holder, with the chip carriers held therein is moved over a substrate containing solder preforms and then vacuum is released. The preforms or bumps are then heated to form a solder bump on pads of the chip, allowing it to reflow between the chip and the desired location, where other bumps similarly flow.
In U.S. Pat. No. 4,097,986 to R. Henry et al., there is described a technique of bonding the underside of a diode to a thin substrate, although solder bumps are not used. In this patent, the substrate receiving the chip is specially prepared by patterned etching layers deposited on the substrate and depositing material in the layers to join with the chip.
One prior art approach has been devised in which no special holders nor substrate preparations were required. Each chip was optically aligned with a proper position on a board by means of a beamsplitter cube, inserted between each chip and the circuit board. The board was moved until the images of the chip's solder bump pattern and the circuit board's solder bump pattern coincided. The beamsplitter cube was then swung out of the way, and the chip lowered onto the circuit board. Even when a chip was properly seated in this manner, it was easy to dislodge when seating a neighboring chip. Unusual skill was required to serially place a large array of chips on a board in a short enough time that the solder flux did not dry out before the assembly could be placed in an oven for solder bump reflow to complete the joinder process.
A problem in connection with prior art solder bump methods arises in trying to remove a single chip on a board or substrate for replacement, if defective or in the event of chip failure. It is quite difficult to melt solder bumps under one chip, and to dress the area for its replacement.
An object of the invention is to devise a method for quickly seating integrated circuit chips of the type adapted for direct joinder onto a circuit board or substrate, without modification of the substrate and without special holders for the chips. Another object was to devise a method of replacing defective such chips.